In all radio receivers, the first amplifier after the antenna when entering the receiver should be especially low-noise type, because the signal level at the input of this amplifier is very low, and the additional noise caused by the amplifier is amplified in all the following amplifier stages. An abbreviation LNA is generally used of such a low-noise pre-amplifier. So in this description and the claims, too. Some allowed maximum value is generally specified in receivers for the total noise figure of the LNA and its input and output circuits. Losses on the transmission path cause attenuation in the signal, directly increasing the noise figure by the same amount. Therefore, for example, if the antenna filter of the receiver is very low loss, the noise figure of the LNA can be correspondingly a little higher.
FIG. 1 shows a block diagram of a common structure of the antenna side part of a receiver. In addition to the antenna and a possible antenna switch, the structure includes an antenna filter, signal divider, two parallel amplifier branches and a signal combiner. In the example of the figure, the antenna filter RXF has two parts: starting from the antenna, there is first a bandpass filter 110 and then a low-pass filter 120. The former attenuates frequency components outside the receiving band of the radio system, and the latter further cleans up the range above the receiving band. The signal Ein coming from the low-pass filter 120 is divided in the divider 130 into two mutually identical parts E11 and E21. The first division signal E11 is led to the first amplifier branch, where its phase is changed 90 degrees in a phaseshifter 140 and then amplified in the first LNA 170. The input impedance of the amplifier must naturally be matched, for which reason there is the first matching circuit 150 in its input. The first LNA outputs the signal E12. The second division signal E21 is led to the second amplifier branch, where it is amplified in a second LNA 180, in the input of which there is the second matching circuit 160. The phase of the signal is then changed 90 degrees in the second phase shifter PSC, which outputs the signal E22. Again, the in-phase signals E12 and E22 are summed in a combiner CMB, the output signal of which, Eout, continues towards the mixer of the receiver, In addition, FIG. 1 shows also amplifier output matching circuits, which do not fall within the scope of this invention, as blocks M. Compared to a single LNA, in the arrangement described above especially the impedance matching of the antenna filter towards the amplifiers is easier. In addition, a wider dynamic and linear area and a better stability are achieved. On the other hand, the divider, the phase shifter and the additional wiring required by them cause more attenuation in the signal, which, as mentioned, directly impairs the noise figure of the LNA.
In this description and the claims, the name “front stage” is used for the parts of the receiver from the antenna to the low-noise amplifiers, including these.
FIG. 2 shows an example of a known input arrangement of an amplifier pair according to FIG. 1. It comprises a circuit board 201, the lower surface of which, not visible in the figure, is conductive and functions as the signal ground GND. The integrated antenna filter RXF comprises resonators, and its output is connected through a connector 225 on its end wall to a coaxial cable 229, which has a characteristic impedance of 50Ω. The conductive cable sheath is connected to the signal ground at both ends. The cable 229 continues on the circuit board 201 as a transmission line, which consists of a micro strip 231 on the upper surface of the board, a ground conductor on the lower surface and dielectric material between them. The transmission line is dimensioned so that its characteristic impedance is 50Ω. It belongs to the divider 230 as its input line. The divider is of the Wilkinson type, which means that the above mentioned input line branches into two transmission lines, which are called division lines here. Their length is λ/4 on the operating frequency, and their characteristic impedance is √2·50 ≈71Ω. The first division line is formed of the first division conductor 232 on the upper surface of the board, the ground conductor on the lower surface and dielectric material between them, and the second division line correspondingly of the second division conductor 233 on the upper surface of the board, the ground conductor on the lower surface and dielectric material between them. A Wilkinson divider is formed when the tail ends of the first and the second division conductor have been connected together by a resistor 234 of the value of 2·50 =100Ω. In that case, if both transmission line branches have been terminated by an impedance of 50Ω, the energy coming from the filter is divided into them half and half, and theoretically without losses. Thus the divider does not consume energy in spite of the resistor in it. Only if the matching on the transmission paths continuing forward is inadequate, the resistor 234 causes losses. In addition, a good isolation between the branches is achieved. The first division line continues as a phase shifter, which has been implemented with a quarter-wave long transmission line. A micro strip 241 of this transmission line, which is a continuation of the first division conductor 232, is seen in FIG. 2. That micro strip ends in the first matching circuit 250 including an air core coil L1 and a chip capacitor C1 in series. The latter functions as a decoupling capacitor at the same time. The matching circuit is connected at its tail end with a short micro strip to the input pin of the first LNA 270. The second division conductor 233 is connected at its end on the side of the resistor 234 to the second matching circuit including a coil L2 and a capacitor C2 in series in the same way as in the first matching circuit. The second matching circuit is connected at its tail end with a short micro strip to the input pin of the second LNA 280.
The arrangement according to FIG. 2 has the drawback of losses that occur in it in practice: the circuit board material causes dielectric losses both in the divider 230 and the phase shifter, the value of the losses being typically 0.2-0.5 dB in the former and 0.1-0.3 dB in the latter. The transmission line 229 from the filter to the divider and its connectors cause additional losses, the value of which can be several tenths of a decibel, naturally depending on the length of the line. The losses of the matching circuits on the input side of the amplifiers are also significant. In addition, the coil of the matching circuit causes a production problem, because the variation of its inductance is so wide in practice that the impedance matching on the operating band may be insufficient. This means additional losses in the divider. Attenuation corresponding to all losses directly increases the noise figure of the amplifier unit by the same amount. Then the requirements for the LNA itself correspondingly increase, if the total noise figure must remain as low as possible.
FIG. 3 shows another example of a known input arrangement of an amplifier pair according to FIG. 1. This differs from the arrangement of FIG. 2 only for the low-pass filter, otherwise the circuit is similar. In this example, the low-pass filter 320 consists of a conductor area on the upper surface of the circuit board 301 and the planar signal ground of the lower surface. The conductor area consists of a straight and relatively narrow micro strip 321, which extends from the input of the filter to its output and in which the substantial characteristic is its inductance. The micro strip 321 has transverse enlargements on, such as an enlargement 322, the substantial characteristic of which is their capacitance in relation to the ground plane. The structure thus corresponds to an LC chain implemented by discrete components, with coils in series, and a capacitor connected to the ground between each two coils. In the example of FIG. 3, there are four “coils” and three “capacitors”, in which case the order of the low-pass filter is seven. The values of the inductances and the capacitances naturally depend on the dimensioning of the parts of the conductor area, which dimensioning thus determines the filter response. The micro strip 321 of the filter 320 continues as micro strip 331, which together with the ground on the lower surface of the circuit board and the dielectric material between them forms the input line of the Wilkinson divider 330. In order to improve the mutual matching of the filter 320 and the divider 330, there is a capacitor 307 at their junction, between the micro strip on the upper surface of the circuit board and the ground.
Because of the filter solution, the arrangement of FIG. 3 is more compact than the arrangement of FIG. 2. The cabling does not cause losses in this case, but a new drawback is caused by the dielectric losses that arise at the low-pass filter in the circuit board. Here, like in the example of FIG. 2, the losses can be reduced by selecting a low-loss material, such as teflon, instead of a generally used circuit-board material. However, in that case there is a flaw of a significant increase in production costs.